Unchained Monk penalty for multiple weapons

[dlaciv12]

I'm working on an Unchained Monk and the FoB desciption says:

He takes no penalty for using multiple weapons when making a flurry of blows, but he does not gain any additional attacks beyond what’s already granted by the flurry for doing so.

Is there some way to activate FoB? I get the -4 penalty when I equip 2 monk weapons.